Octopuses, squids, and similar animals are members of a group of ocean invertebrates (animals with no backbone) called cephalopods. Cephalopods are a kind of mollusk, a group of animals that also includes clams, oysters, and snails. The name "cephalopod" means "head-foot," which refers to the fact that the limbs of these animals are attached to their head.

Cephalopods live throughout the world's oceans, from surface waters to depths of more than 4 miles (7 kilometers). About 700 different species (types) exist. The largest is the giant squid, which measures up to 60 feet (18 meters) long. The smallest is the pygmy squid, which measures just 1/2 inch (1.27 centimeters) long.

All cephalopods have at least eight arms, and sometimes two or more feeding tentacles. The arms are usually covered with powerful suckers, while the longer tentacles may have suckers all along their length or only at their tips. These suckers help the animals catch fish and other prey. In some species the arms also have hooks. All cephalopods have a beak, similar to a bird's beak, that is used to bite prey. Cephalopods have well-developed eyes, and some species use their keen vision to hunt for food.

Expert hunters, cephalopods are also skilled at avoiding predators. Their skin is covered with chromatophores, pigment-filled sacs that contract or expand to change the animal's color to match its surroundings. (Cephalopods also communicate with each other by changing color.) Many cephalopods squirt a black, ink-like substance into the water to confuse predators.

A cephalopod's internal organs, which include three hearts, are protected by a fleshy covering called the mantle. Inside the mantle is the mantle cavity, where gills pick up oxygen from the water. After the water passes over the gills, it can be ejected forcefully from the mantle cavity through a funnel. This propels the animal through the water.

Kinds of CephalopodsAmong the many different kinds of cephalopods, five groups are the most notable. In addition to octopuses and squids, the other groups are the nautiluses, the cuttlefishes, and the vampire squid.

OctopusesOctopuses are found in every ocean, and there are more than 100 species. They use their eight muscular arms to crawl along the ocean floor. They lack feeding tentacles, and most lack fins. The giant octopus may weigh more than 220 pounds (100 kilograms) and measure about 15 feet (5 meters) from the tip of one arm to the tip of the opposite arm.

Octopuses hide in cracks and crevices in the rocks or in burrows in sand and mud. Seizing their prey with their arms, they inject it with a paralyzing toxin before eating it. This toxin also starts the process of digestion. A female octopus attaches her eggs to rocks and guards them until they hatch; then she dies. Octopuses have well-developed brains, and they are considered the most intelligent invertebrate.

SquidsSquids have eight arms, two long feeding tentacles, and a fin on each side of their body. They are found in every ocean and spend their lives swimming, floating, or sometimes resting on the bottom. Squids that live in shallow water are slender and streamlined, and they are fast and active hunters. Deepwater squids tend to be more delicate and more sluggish; they spend most of their time floating in the dark, waiting for food to come to them. Although squids can move by shooting water from their mantle cavity, they also use their two fins to swim.

Some species of squids attach their egg masses to the ocean floor, while others lay egg masses that float. One deepwater species carries its eggs in its arms until they hatch. Most squids die after laying their first batch of eggs.

NautilusesThe chambered, or pearly, nautilus is a shelled cephalopod. Five species live in the tropical waters of the Indian and Pacific oceans. Nautiluses have about 90 arms, none of which have suckers. They rest in deep water during the day, but at night they rise into shallow water to feed. Although most cephalopods live for only a short time, nautiluses may live as long as 20 years. Their beautiful, coiled shells are popular with collectors.

CuttlefishesThere are more than 100 species of cuttlefishes, which live in the Mediterranean Sea and in many oceans of the world. Like squids, these animals have eight arms, fins on each side of their body, and two tentacles. Cuttlefishes are usually less active than squids. They live on the ocean floor or float just above it in shallow water. Cuttlebone, a chalky, shell-like structure inside the body, helps cuttlefishes float.

Vampire SquidsThere is only one known species of vampire squid, which is not a true squid. It lives in deep waters around the world. This animal is named for its black skin and for the webbing between its arms that looks like a vampire's cape. It lacks feeding tentacles , but unlike other cephalopods, it has sensory filaments that can be extended to about five times its body length to feel for food. The vampire squid is about 15 inches (38 centimeters) long and has the largest eyes of any animal in relation to the size of its body. Vampire squids are very sluggish swimmers, primarily using their paddle-shaped fins to move through the water. They have an unusual trick for confusing predators: When they are disturbed, special organs near their fins and at the tips of their arms produce light, through a chemical process called bioluminescence.

Cephalopods and PeopleMost cephalopods are not dangerous to people. There are some notable exceptions, however. The venom of the blue-ringed octopus can be fatal to humans. The Humboldt squid, which can grow as long as 6 feet (2 meters), will attack scuba divers, although no serious injuries have occurred.

Humans, on the other hand, can be very dangerous to cephalopods. Octopuses, cuttlefishes, and squids are popular food items in many parts of the world and support large commercial fisheries. If too many of these animals are caught before they can reproduce, their populations could decline very rapidly.